1. Field of the Invention
The present invention relates generally to an organic positive temperature coefficient thermistor, and more particularly to an organic positive temperature coefficient thermistor that makes use of a phenomenon wherein its resistance value increases sharply with increase in a certain temperature, i.e., PTC (positive temperature coefficient of resistivity) characteristics.
2. Description of the Background
An organic positive temperature coefficient thermistor prepared by milling and dispersing conductive particles of carbon black, metal, etc. with and in a crystalline polymer and having PTC characteristics has been well known in the art, as typically disclosed in U.S. Pat. Nos. 3,243,753 and 3,351,882. This can be used as a self-regulating heater, a temperature sensor, and an overcurrent-protecting element. Requirements for this thermistor are its sufficiently low initial resistance value at room temperature (in its non-operating state), its large change in its resistance value at an operation temperature, and its stable performance upon repetitive operations.
For a prior art organic positive temperature coefficient thermistor, it is generally known that the crystalline polymer melts upon operation and, hence, the dispersion state of conductive particles changes upon cooling, resulting in increase in its initial resistance value.
For conventional organic positive temperature coefficient thermistors, carbon black is often used as conductive particles. A problem with carbon black is, however, that when a content of carbon black in an organic PTC thermistor is increased to lower the initial resistance value, no sufficient rate of resistance change is obtainable. Sometimes, particles of generally available metals are used as conductive particles. In this case, too, it is difficult to obtain both low initial resistance and a large rate of resistance change.
One approach to solve this problem is disclosed in JP-A 5-47503 that teaches the use of conductive particles having spiky protuberances. More specifically, it is disclosed that polyvinylidene fluoride is used as a crystalline polymer and spiky nickel powders are used as conductive particles having spiky protuberances, whereby a reasonable compromise can be reached between low initial resistance and a large resistance change. However, the thermistor set forth in this publication is less satisfactory in terms of performance stability upon repetitive operations. When the polyvinylidene fluoride is used, the operating temperature is about 160.degree. C. However, if operating temperatures exceeding 100.degree. C. are applied to such applications as protective elements for secondary batteries, electric blankets, toilet stool heaters, seats for vehicles, etc., they are extremely hazardous for the human body. To safeguard the human body, the operating temperature must be below 100.degree. C., especially about 60 to 70.degree. C.
U.S. Pat. No. 5,378,407, too, discloses a thermistor comprising filamentary nickel having spiky protuberances, and a polyolefin, olefinic copolymer or fluoropolymer, and alleges that this thermistor has low initial resistance, a large resistance change, and good-enough performance stability upon repeated operations. With highdensity polyethylene, and polyvinylidene fluoride polymers used in the examples, the operating temperatures are about 130.degree. C. and about 160.degree. C., respectively. Although this publication states that use may also be made of ethylene-ethyl acrylate copolymers, ethylene-vinyl acetate copolymers, ethylene-acrylic acid copolymers, etc., it fails to give examples using the copolymers. With these copolymers, the operating temperature of thermistors can be 60 to 70.degree. C. As will be seen from examples to be given later, however, the performance of the thermistors upon repetitive operations is found to be unstable.
U.S. Pat. No. 4,545,926 discloses a similar thermistor comprising a spherical, flaky or rod form of nickel, and a polyolefin, olefinic copolymer, halogenated vinyl or vinylidene polymer. Some exemplified thermistors based on ethylene-ethyl acrylate copolymers, and ethylene-acrylic acid copolymers have an operating temperature of 60 to 70.degree. C., and some exemplified thermistors based on other polymers have an operating temperature exceeding 100.degree. C. However, the performance of thermistors based on ethylene-ethyl acrylate copolymers, and ethylene-acrylic acid copolymers are unstable upon repetitive operation, as already noted.
On the other hand, JP-A's 62-65401, 62-122083, 62-156159, 2-72580, 2-172179, 3-187201 and 4-14201 disclose thermistors comprising carbon or conductive metals, and polyethylene oxides. These thermistors may be operated at 60 to 70.degree. C. However, it is impossible to obtain both low initial resistance and a large resistance change as already mentioned, because carbon, and conductive metals are used.
All these polyethylene oxides have molecular weights of 100,000 or lower. It has now been found that even when such polymers are used in combination with filamentary nickel having spiky protuberances as disclosed in U.S. Pat. No. 5,378,407, it is impossible to obtain a thermistor that has low initial resistance and sufficiently sharp rise in resistance at an operating temperature.
Thus, never until now is an organic positive temperature coefficient thermistor achieved, which can show good performance at an operating temperature of less than 100.degree. C. that is little hazardous to the human body, and have high performance stability.